Spectrographs (sometimes referred to as spectrometers) are common instruments used to measure the properties of input light across the component wavelengths of the input light, e.g., the intensity of the light at some or all of the component wavelengths of the input light. They are particularly useful in the fields of material and chemical analysis, where light of different types (infrared, visible, and/or ultraviolet) may be directed onto a sample, and the resulting light reflected by, emitted by, and/or transmitted through the sample can then be supplied to and analyzed by the spectrograph. The resulting readings can provide information about the properties of the sample.
A common spectrograph design incorporates a monochromator, a device which separates the input light into its component wavelengths, and a detector (usually a photosensor) which measures light intensity at one or more of the wavelengths. One type of monochromator, known as a Czerny-Turner monochromator, is depicted in the accompanying FIG. 1 at the reference numeral 100, and has three main components: a diffraction grating 102, a primary reflector 106 (a mirror, usually spherical or toroidal, supplying the grating 102 with light), and a secondary reflector 104 (a mirror, usually spherical or toroidal, receiving light from the grating 102). An aperture 108 (shown as a narrow slit, though it could be a pinhole instead) at the focus of the concave primary reflector 106 admits input light (e.g., light reflected or emitted by a sample to be analyzed), with the input light then being incident on the primary reflector 106. The primary reflector 106 collimates the light rays within the beam (reorients the rays into parallel paths focused at infinity), with the collimated beam then being directed to the diffraction grating 102. The grating 102 has an array of fine slits (not shown) arrayed across its face. (While termed “slits,” these are usually not slits in the sense that they define apertures across the face of the grating 102; rather, the slit array is generally defined by an array of angled surfaces, e.g., the face of the grating 102 may have a sawtooth or sinusoidal profile on a microscopic or near-microscopic scale.) The grating 102 reflects light at different wavelengths at different angles, with FIG. 1 depicting rays at three different wavelengths by phantom/dashed lines having dashes of different sizes and spacings. All or most of the reflected light is then received by the concave secondary reflector 104, which focuses the light at each wavelength onto some output element such as an array detector 110 (e.g., an array of photosensitive elements) which measures light intensity at each of the component wavelengths. Thus, a user obtains a measurement of light intensity across the range of wavelengths incident on the detector 110, and the identities of these wavelengths may be calculated with knowledge of the characteristics of the primary reflector 106, grating 102, and secondary reflector 104 (and the location of the readings across the detector 110).
However, the arrangement of FIG. 1 can suffer from aberrations common to all non-ideal optical systems, such as coma, astigmatism, and spherical aberration. These aberrations result in distortion of the image on the detector 110 and therefore limit the resolution of the spectrograph. For example, spherical aberration, an aberration inherent in optical elements having spherical surfaces (such as in the spherical reflectors 104 and 106), can also result in some rays being out-of-focus at the detector 110 and can hinder resolution. More significantly, coma—an aberration caused by the reflection of light rays at nonperpendicular angles, wherein the rays elongate in the plane of the incident and reflected rays—cause the different-wavelength rays to at least partially overlap on the detector 110 rather than resting in separate bands, thereby affecting the accuracy of the intensity readings. Careful choice and alignment of the primary and secondary reflectors 106 and 104 can reduce coma across a range of wavelengths, with coma correction being optimal at a selected “design wavelength” and decreasing at wavelengths away from the design wavelength. As a result, the arrangement of FIG. 1 is practically limited for use at its design wavelength and for surrounding wavelengths. If a different range of wavelengths is desired, one must change the angle of the grating 102, or use a different grating 102. But these would in turn require reorientation of the primary and secondary reflectors 106 and 104, as well as reorientation and/or relocation of the detector 110, in order to reduce aberration about the new design wavelength. In practice, this is difficult and/or expensive to accomplish because of the complexity in situating the various optical elements (which must be rather precisely placed) and the need for the various positioners/actuators and controls required to accomplish placement of the optical elements.
It would therefore be useful to have a monochromator arrangement which allows high-resolution dispersion of input light into its component wavelengths over a wider range of wavelengths, and/or which more readily accommodates changes in gratings or grating angles.